WAVELENGTH LOCKING AND POWER CONTROL SYSTEMS FOR MULTI-CHANNEL PHOTONIC INTEGRATED CIRCUITS (PICs)

ABSTRACT

A transmissive active channel element is provided in each signal channel of a monolithic multi-channel TxPIC where each channel also includes a modulated source. The active channel element functions both as a power control element for both monitoring and regulating the output channel signal level of each signal channel and as a modulator for channel wavelength tagging or labeling to provide for wavelength locking the modulated sources. The power regulating function is also employed to control the channel signal power outputs of each channel to be uniform across the channel signal array. All of these functions are carried out by a feedback loop utilizing digital signal processing.

REFERENCE TO RELATED APPLICATION

This application claims priority to provisional application, Ser. No.60/695,382, filed Jun. 30, 2005, and is also a continuation-in-part ofnonprovisional patent applications, Ser. No. 10/267,330, filed Oct. 8,2002 and entitled, TRANSMITTER PHOTONIC INTEGRATED CIRCUIT (TXPIC) CHIPARCHITECTURES AND DRIVE SYSTEMS AND WAVELENGTH STABILIZATION FOR TxPICs,which claims priority to provisional application Ser. No. 60/370,345,filed Apr. 5, 2002; and Ser. No. 10/267,331, filed Oct. 8, 2002 andentitled, TRANSMITTER PHOTONIC INTEGRATED CIRCUITS (TXPIC) AND OPTICALTRANSPORT NETWORKS EMPLOYING TxPICs, which claims priority toprovisional application, Ser. No. 60/328,207, filed Oct. 9, 2001, all ofwhich applications are incorporated herein by their reference in theirentirety.

BACKGROUND OF THE INVENTION

This invention relates to wavelength locking and output power controlsystems relative to multiple WDM signal channels and more particularlyto such signal channels as found in a monolithic transmitter photonicintegrated circuit (TXPIC) chip having a plurality of integrated signalchannels with each channel having a modulated source.

There are many feedback loop systems known for controlling thewavelength of an array of lasers, particularly discrete lasers or socalled EMLs employed in an optical transmitter for use in an opticaltransport network. Also, there are feedback loop systems to control theoutput level of modulated signals produced in such transmitters so thattheir power levels are equal across the array of signal channelgenerators. This power equalization is also referred to in the art aspre-emphasis. A characteristic of a monolithic TxPIC with an integratedmultiplexer is that the light that emerges from the TxPIC alreadycombines a plurality of data-modulated optical wavelengths. Whileadvantageous for reliability and reduction of cost, this integratedmultiplexing function poses challenges for control of individual opticalwavelengths and channel average powers, as the information required tocontrol individual channel powers and wavelengths must be extracted fromthe optical multiplexed signal at the output of the TxPIC.

What is needed is a control system that can concurrently controlemission wavelengths and powers on such an array of lasers and moreparticularly on an array of integrated lasers or modulated sources in atransmitter photonic integrated circuit (TXPIC) with an integratedoptical multiplexer. The control system can advantageously employintegrated channel active elements to aid in such emission wavelengthand signal output power control.

SUMMARY OF THE INVENTION

The channel wavelength and power control system of this disclosureprovides principally three functions:

1. The locking of wavelengths of modulated sources of a multi-channelintegrated TxPIC to a standardized wavelength grid employing a sharedwavelength reference.

2. The prevention of wavelength locking to incorrect reference valuesunder various aging, re-starting and channel failure scenarios.

3. The detection and controlling of the individual signal channel powersof a plurality of modulated sources in a monolithic transmitter photonicintegrated circuit (TxPIC).

An important feature of this disclosure is employment of a PIC signalchannel-specific tagging or labeling scheme and method that serves thedual functions of channel wavelength locking and channel power control.

Also, an important feature of this disclosure is the deployment of acontrollable, transmissive active channel element provided in eachsignal channel of a multi-channel transmitter photonic integratedcircuit (TxPIC). By “transmissive”, we mean that the element istransparent to a channel signal from a modulated source propagatingthrough the element. The controllable, transmissive, active channelelement functions both as 1) a modulator for labeling each signalchannel with an optical intensity modulation of known modulation depthand frequency to provide channel characteristic optical modulation tag,and 2) a power control element for regulating the output channel signallevel of each TxPIC channel. Detection of attributes of an individualoptical channel within an optical multiplexed signal is accomplished bydetecting the strength of that channel's characteristic opticalmodulation tag. The feedback loops for both channel wavelength andchannel power control provide for parallel control for all on-chipsignal channels, i.e., each of the PIC channels on the TxPIC is providedwith a modulated tone tag simultaneously and these tags are detectedsimultaneously in the output of a given photodetector receiving acomposite signal from the optical multiplexed signal. Parallel signalprocessing of all channels in a multiplexed signal is important for anintegrated PIC device because a change in channel wavelength and powercan immediately affect the emission wavelengths and powers ofneighboring signal channels. This effect is especially pronounced if anadjacent channel modulated source expires or dies. Thermal coupling ofadjacent channels will have rapid, dynamic effect on the emissionwavelengths of adjacent channels. Thus, a feedback control system mustbe able to respond rapidly, such as within one millisecond. Parallelsignal processing, based on simultaneous demodulation of the tone tagsplaced on all signal channels, enables fast control of the channelwavelengths and powers in this coupled, multichannel system

In the multi-channel integrated TxPIC, each signal channel includes adata-modulated source, defined as a directly modulated laser or acw-operated laser with an external modulator. The transmissive activeelement of each channel may be, as one example, a waveguide PIN region,the transmission of which can be varied according to the reverse biasvoltage (varying the absorption of the PIN) or the forward bias current(varying the gain of the PIN). A PIN region is an intrinsic regionbounded by a p-type and n-type confinement region. For descriptionpurposes, this waveguide PIN region is hereinafter referred to as a“front PIN”. However, as will readily be understood by those skilled inthe art, the transmissive active element may be another channeltransmissive active element, examples of which are provided later one inthis disclosure. Each channel further includes a back photodetector(PD), whose purpose is to absorb substantially all of the light emittedfrom the backend of the channel laser source. At the time ofmanufacture, the back PD is employed to measure the photocurrentassociated with a beginning of life (BOL) output power of each channellaser and a simulated end of life (EOL) output power of each channellaser. The ratio of laser forward output power to backward output poweris substantially constant over life. Thus, the back PD photocurrentreadings are a fairly good indicator of the forward light internal poweroutput from each of the respective channel lasers on a TxPIC. SimulatedEOL power is selected based upon estimation of laser degradation overlife which is normally a few dB down from its BOL power output, such as,for example, between about 1.5 dB to about 3 dB of power outputdegradation over life. For each of the two selected optical output powerlevels from the channel laser source, the forward output powertransmitted through the front PIN as a function of its bias state isdetected to produce two curves referred to as transfer functions forthat channel's front PIN. The beginning-of-life (BOL) transfer functioncurve represents the normalized transmittance versus reverse biasvoltage of the front PIN at the BOL laser power state, for which theBack PD current is also known. The simulated end-of-life (EOL) transferfunction curve represents the normalized transmittance versus reversebias voltage of the front PIN at the simulated EOL laser power state,for which the Back PD current is also known. During operation, thefollowing technique is used to estimate the normalized transfer functionassociated with the output power of the channel laser source. Thecurrent of the back PD is read, and its value relative to the known BOLand simulated EOL values is determined. Using the detected Back PDcurrent as an interpolation parameter, linear interpolation between thenormalized BOL and EOL normalized transfer functions is performed Thisinterpolated transfer function is then employed to determine how to setthe transmission of the front PIN to achieve a desired value for thechannel output power. With the transfer function known, it is possibleto set an average desired transmittance (using an appropriate averagebias setting). It is also possible to introduce an intensity modulationof known modulation depth and desired average transmittance by selectingan appropriate AC modulating waveform for the bias of the front PIN. Inthis way, it is possible both to label an optical channel with anintensity modulation of known optical modulation index while alsocontrolling the average power of the channel, using a singletransmissive active element per channel.

To label the individual channels within an optical multiplexed signal,carefully selected intensity modulation waveform schemes can be used.One such labeling scheme uses square waves whose fundamental frequenciesand phase relationships are chosen to produce approximate mathematicalorthogonality of the product of any pair of different square waves overa particular integration time interval. In this scheme, each channel isintensity modulated by applying the appropriate square wave “tone” tothe bias input of the channel's front PIN. The fundamental frequency ofthe square wave tone is unique to its assigned channel in themultiplexed signal, and the depth of optical modulation is made to beknown and constant based on the calibrated transfer function(transmittance versus bias) of the channel's front PIN. All channels inthe multiplexed signal of interest are intensity modulated in parallel,with waveforms synchronized to maintain optimal orthogonality. Detectionof the attributes of an individual channel in the multiplexed signalinvolves demodulation of the composite detected signal using thechannel's assigned label. Consider the output of a photodetector thathas as its input a tapped portion of the optical multiplexed signal. Thephotocurrent will contain, among other things, AC signals associatedwith the constant optical modulation index intensity modulation labelsfor each channel in the multiplexed signal. The composite photocurrentsignal can be converted to a voltage and densely sampled by an analog todigital converter to enable subsequent digital signal processing. Toextract the information associated with a particular channel, theproduct of the sampled composite signal and the channel's selected andsynchronized square wave is created and integrated over a period chosento provide approximate orthogonality among different square waveslabeling channels in the multiplexed signal. In this way, the portion ofthe composite signal that corresponds to the square wave labeling theindividual channel of interest is extracted by digital signalprocessing, Owing to the approximate mathematical orthogonality of theselected square waves, the output of the integration process is a singlenumber primarily determined by the strength of the tone labeling ortagging of a channel under test. Cross-correlation terms from all theother channels can be made sufficiently small to be negligible. Theprocess of sampling the composite received signal and integrating withan appropriately synchronized version of the tone used to label thechannel under test over the integration interval that preservesorthogonality is referred to as demodulation of that channel's label ortag. All channels in the multiplexed signal can be demodulated inparallel by submitting the sampled composite output signal from thephotodetector to parallel integration processes, one for each label tonein the orthogonal labeling scheme. For an individual channel, the resultof demodulation is a number indicative of the size of the AC labelsignal at the receiver. This AC label signal is proportional to theproduct of the average optical power of the channel the opticalmodulation index of the channel. If the optical modulation index of thechannel is known, one can then readily deduce the average opticalchannel power. The advantage of this technique is that AC signal labelsand AC signal processing are used deduce individual channel powerswithin a multiplexed signal, without requiring that the individualchannels be separated out, such as through optical demultiplexing, forexample, to allow detection of individual DC photocurrents. So, N signalcorrelations are concurrently applied for all N channels, simultaneouslyindicating the average power in each signal channel for N channels,provided the optical modulation index of each channel is known amongpossible labeling waveforms, square wave modulation is employed becausethe modulation frequencies can be carefully selected to exhibitnear-zero cross-correlations over the integration interval and themodulation is easier to implement via digital switching,digital-to-analog converters (DACs) are employed to set both the voltagehigh level, V_(High), and the voltage low level, V_(Low), signalsapplied to the channel active element, and the digital output of an FPGAtoggling between two voltages at the selected modulation frequency candrive an analog switch to generate the square wave modulation at thechannel transmissive active element for each signal channel on theTxPIC. However, it should be noted, as will be evident by those skilledin the art that, for example, the waveform used for channel activeelement modulation could have been chosen to be sinusoidal.Orthogonality properties of sine waves are particularly well known.

Summarizing, the front PIN of each channel on the TxPIC serves twopurposes. First, the front PIN serves as a controllable attenuator orgain element for the channel. Appropriate calibration of the normalizedtransfer function (transmittance versus bias) for the front PIN providesthe information needed to determine the desired average bias state,given a target output value for the channel and a measurement of theactual output power of the channel. Second, the front PIN serves as amodulator to label the individual channel with an intensity modulationlabel or tag chosen to satisfy signal processing requirements includingestablishing and maintaining a known optical modulation index for eachchannel. Using the orthogonality properties of the selected channellabels, correlation techniques can be used to extract, from a compositephotocurrent associated with detection of the multiplexed signal, theaverage optical powers of each of the individual channels making up themultiplexed signal. Therefore, a control system for the powers ofindividual optical channels within a multiplexed signal can be based ondetecting a tapped fraction of the multiplexed signal with a simplephotodetector and carrying out the demodulation process described above,together with the calibrated relationship between average bias andtransmittance of the front PIN. In one version of this control system,an optical tap and photodetector can be used to route a fraction of theoptical multiplexed signal emerging from the monolithic TxPIC withintegral multiplexer. Demodulation of the output of the photodetector asdescribed above allows measurement of the channel powers arriving at thephotodetector, and feedback to the individual front PINs on the channelsof the TxPIC allows the individual channel powers to be set to desiredvalues. For example, the channel powers can all be made to beapproximately equal at the photodetector. Alternatively, differentchannels may be controlled to different output power setpoints ifdesired.

The description of the demodulation process so far has addressed onlythe issue of power control for individual channels of a TxPIC. Inconjunction with an appropriate form of wavelength reference, thedemodulation signal processing can also be used to control the channelwavelengths of individual channels of a TxPIC.

The use of channel labeling and demodulation as described above inconnection with detection of average optical channel powers ofindividual channels within an optical multiplexed signal can be extendedto the application of wavelength locking as follows. A device such as aFabry-Perot etalon can be manufactured such that its free spectral range(frequency spacing between adjacent transmission peaks) is selected tocorrespond to a desired frequency spacing such as may be associated withthe conventional ITU frequency grid. For example, the free spectralrange of the Fabry-Perot etalon might be selected to be 50 GHz. Throughwell-known optical alignment procedures, the transmission peaks of theFabry-Perot etalon can be arranged such that each specific frequencyassociated with a 50 GHz ITU frequency grid is associated with a pointroughly halfway up and along the side of a transmission fringe. Theimportant result of this alignment process is that, for an opticalcarrier whose frequency is close to an ITU grid frequency, thetransmission of the etalon depends on the optical carrier frequency;i.e. the ratio of transmitted power to incident power for the particularchannel depends on the channel's optical carrier frequency and the localslope of the associated fringe of the Fabry-Perot etalon. In simpleterms, the Fabry-Perot etalon can, therefore, provide an opticalfrequency discrimination function for any optical carrier frequency forwhich the desired setpoint corresponds to a point on the sloped side ofa transmission fringe.

For a single optical channel, a sensor for detecting deviation of theoptical carrier frequency can be developed by dividing or splitting atapped portion of the multiplexed signal output from the TxPIC andsending the split signal portions through two different optical pathsterminated by photodetectors. One path (the etalon path) causescollimated light from the optical channel to pass through the alignedFabry-Perot etalon, which provides a frequency-discriminating functionsuch that the transmitted power depends on the optical carrierfrequency. The other path (the reference path) contains no opticalfrequency discriminator elements and simply provides a measure of theaverage power of the optical channel. Comparing the output photocurrentsfrom these two paths allows one to measure the optical carrier frequencyof the channel relative to its position on the associated Fabry-Perotetalon transmission fringe. For example, the ratio of the photocurrentfrom the etalon photodetector to the photocurrent from the referencephotodetector provides a unique measurement of the optical carrierfrequency that is independent of the optical channel power. Commercialdevices such as a broadband Fabry-Perot wavelength locker which isavailable from JDSU, Inc. are well known in the art.

The application of the Fabry-Perot wavelength locker (or equivalent) forproviding a measurement of the optical carrier frequency of a singleoptical channel has thus far been described in terms of detected DCphotocurrents. If an optical multiplexed signal containing N multiplesignal channels is routed to such a Fabry-Perot wavelength locker, theDC photocurrents can no longer be used to provide useful informationabout the optical carrier frequencies of the channels. Usefulinformation can, however, be derived from the composite photocurrents ofthe etalon and reference photodetectors if the individual channels ofthe optical multiplexed signal are labeled by intensity modulationwaveforms, as described previously, and if the outputs of both theetalon and reference photodetectors are sample and demodulated inparallel. For a given labeling tone frequency, the result ofdemodulation of the etalon photodetector signal will be a numberproportional to the average optical power of the channel labeled by thattone frequency, the optical modulation index of the channel, and theoptical carrier frequency of the channel (owing to the opticalfrequency-discriminator characteristic of the sloped side of an etalontransmission fringe). Similarly, the result of demodulation of thereference photodetector signal will be a number proportional to theaverage optical power of the channel labeled by that tone frequency, andthe optical modulation index of the channel. From this pair of numbersresulting from demodulation, a measurement of the optical carrierfrequency of the channel labeled by the tone frequency can be extractedfrom the composite signal resulting from illumination of thephotodetectors by an optical multiplexed signal comprising multipleoptical channels. Parallel demodulation of the etalon photodetectoroutput and the reference photodetector output at all the tonefrequencies provides a means of simultaneously measuring optical carrierfrequencies of all channels in the optical multiplexed signal, using asingle Fabry-Perot wavelength locker.

Given a calibrated relationship between ITU grid frequencies andtransmission properties of etalon and reference paths of the Fabry-Perotwavelength locker, the measurements of optical carrier frequencies canbe translated to measurements of deviations of optical carrierfrequencies with respect to ITU grid frequencies. Using channel labelingand parallel demodulation as described previously, then, a singleFabry-Perot wavelength locker can be used to measure individual channelcarrier frequencies in an optical multiplexed signal, and to relatethose individual channel carrier frequencies to ITU grid frequencies,provided that each optical carrier frequency remains sufficiently closeto its associated ITU grid frequency (to avoid ambiguities arising fromthe periodic transmission properties of the etalon).

The multiplexed optical output of a multichannel TxPIC, labeled asdescribed in connection with optical channel power control, can berouted to a single Fabry-Perot wavelength locker and processed asdescribed above to measure the individual optical carrier frequencies ofeach channel of the TxPIC. Given a means of altering the optical carrierfrequency of a TxPIC channel, one can complete a control system thatlocks each channel of the TxPIC to an assigned optical carrier frequencyalong a standardized wavelength grid, such as ITU grid frequencies. Onesuch means of controlling the optical carrier frequency of an individualchannel on a TxPIC is a local heater next to the laser that generatesthe optical carrier frequency of the channel.

As previously indicated, while the front PINs of the plurality of signalchannels on a TxPIC are employed to provide dual functions ofmaintaining desired signal channel output power and laser emissionwavelength, it should be importantly noted that other channel activeelements in each PIC signal channel can also be contemplated to providethis dual functionality. Examples of such other channel integratedactive elements are the channel laser source, the channel externalmodulator, a channel variable optical attenuator (VOA), a channelsemiconductor optical amplifier (SOA) or a channel combination SOA/VOA.

While this invention is described as being applicable to the control thewavelength and power between or among a plurality of integrated signalchannels in a monolithic transmitter photonic integrated circuit(TxPIC), it would be readily recognized by those skilled in the art thatthe fundamentals of this disclosure are equally applicable to other WDMsignal channel systems, such as, but not limited to, those opticaltransmission systems having discrete and separate signal channels, suchas, in the case where each signal channel comprises a separate anddiscrete cw laser and corresponding discrete external modulator or adiscrete directly modulated laser, or a discrete but integratedelectro-absorption modulator/laser referred to as an EML. Such WDMsignal channel systems would also include discrete means to combine ormultiplex the signals to provide an optical multiplexed signal outputfrom multiple of such separate modulated sources.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings wherein like reference symbols refer to like parts:

FIG. 1 is a schematic illustration of the wavelength and power controlsystem for a multi-channel transmitter photonic integrated circuit(TxPIC) of this disclosure.

FIG. 2 is a graphic illustration that defines the optical modulationindex (OMI) for a square wave where the optical modulation index μ hereis shown as 0.05 (5%).

FIG. 3 is a graphic illustration of a 50 GHz free spectral range etalontransmission curve as a function of optical frequency showing therelationship between etalon transmission peaks and optical carrierfrequencies for a channel k and a channel k+1 having a 200 GHz gridspacing between signal channels in a multi-channel transmitter photonicintegrated circuit (TxPIC).

FIG. 4 is a graphic illustration of the normalized front PIN transferfunction at constant wavelength, temperature, and input power.

FIG. 5 is a graphic illustration of error determination relative tooptical frequency shift relative to etalon photocurrent for a givenstandardized (ITU) grid frequency.

DETAILED DESCRIPTION OF DISCLOSURE EMBODIMENTS

In FIG. 1, transmitter photonic integrated circuit (TXPIC) 10 may be asemiconductor circuit chip and contains a plurality of N integratedsignal channels 15 where each channel comprises, in a serial train ofelements along each channel, a back photodetector (PD) 12, asemiconductor laser 14 (DFB laser or DBR laser), electro-optic modulator(EOM) 16 (shown here as an electro-absorption modulator or EAM but canalso be, for example, a Mach-Zehnder modulator) and a front PIN 18. Werefer to a signal channel combination of laser 14 and electro-opticalmodulator 16 as a channel modulated source. Another type of modulatedsource that may be employed in channels 15 of TxPIC 10 is a directlymodulated laser which, of course, removes the requirement for anexternal modulator 16 in each channel 11. In TxPIC 10, there are, then,N signal channels 15 with N lasers 14 that have different opticalcarrier frequencies or emission wavelengths that operate along uniformfrequency grid spacing, such as at 50 GHz, 100 GHz, or 200 GHz. Thelaser CW light output is data signal modulated at a channelelectro-optic modulator (EOM) 16.

Only one such channel 15 is shown for purposes of simplicity in FIG. 1but the other N-1 signal channels would be approximately aligned withthe first signal channel with all their outputs optically coupled to theintegrated optical combiner, such as, for example, a multiplexer(arrayed waveguide grating or AWG) 20, which combines the modulatedchannel signal outputs generated at the modulated sources and providesan optical channel group (OCG) multiplexed signal on the outputwaveguide from TxPIC 10. Such a combiner may also be, for example, anEchelle grating (wavelength selective combiner) or an optical coupler,such as an MMI coupler (free space coupler). This PIC output is providedoff-chip to an isolator 22, a tap coupler 24, a variable opticalattenuator (VOA) 26 and on to an output waveguide 28 to another module,indicated here as a band multiplex module (BMM) which is disclosed anddiscussed in US nonprovisional patent application, Ser. No. (P096),filed Jun. __, 2006, a nonprovisional patent application of provisionalapplication Ser. No. 60/695,508, filed Jun. 30, 2005, which applicationsare incorporated herein by its reference. As used herein, OCG is aparticular channel group for a given TxPIC 10, as other TxPICs withdifferent emission wavelength channel groups (OCGs) may also be providedin the same module carrying the embodiment shown in FIG. 1 (referred toas a digital line module or DLM) as TxPIC 10, each with its own feedbackloop 41, and also with their outputs provided to the same BMM onwaveguide 28 where all the OCGs are combined for transport on an opticaltransport network optical link. More about DLMs can also be seen in U.S.nonprovisional application, Ser. No. 11/154,455, filed Jun. 16, 2005,which is also published on Dec. 25, 2005 at US 2005/0286521 A1, whichapplication is incorporated herein by its reference. As an example,there may be N-equal-ten signal channels 15 on TxPIC 10. However, manymore signal channels may be placed on the same TxPIC, such as 40 signalchannels or more. Each signal channel 15 integrated on chip 10 providesa modulated signal output having a different peak wavelength, such as inthe spectral range of the C band on the ITU wavelength grid, whichemission wavelengths are set by each signal channel's correspondingsemiconductor laser 14. As previously indicated, each of these modulatedoutputs is provided to the input of optical multiplexer 20, shown hereas an arrayed waveguide grating (AWG). TxPIC 10 of FIG. 1 is detailed inU.S. patent application Ser. No. 10/267,331, filed Oct. 8, 2002, andalso published on May 22, 2003 as Publication No. US2003/0095736A1,supra.

In FIG. 1, a wavelength locking and power control feedback loop 41 isprovided for multichannel transmitter photonic integrated circuit(TXPIC) 10. One aspect of this disclosure, among several importantaspects, is the employment of a front PIN 18 in each signal channel 15of TxPIC 10 both as power control element to function as a variablechannel attenuator and as a low frequency tone modulator for channellabeling or tagging used in both channel power control and channelwavelength locking. Channel tagging information is employed in thefeedback loop 41 for both wavelength and power control, i.e.,respectively, to control the on-chip channel laser emission wavelengthsand to control channel signal outputs in each signal channel 15, e.g. toachieve equalized channel signal power outputs across the signal channelarray on TxPIC 10. These processes are accomplished by digital signalprocessing as described previously. Thus, an integrated channel frontPIN 18 for each channel functions both as an attenuator to set averageoptical power output of each channel as determined by adjusting theaverage reverse bias voltage per signal channel and as a transducer forintensity modulation of the optical signal received from channelmodulated sources. The intensity modulation chosen here is square waveand is produced by toggling the reverse bias voltage applied to eachfront PIN 18 between two values of reverse bias voltage which arereferred to herein as V_(High) and V_(Low) so that the voltagepeak-to-peak of the modulated square wave is V_(High) minus V_(Low) asbest illustrated in FIG. 4 and is also depicted in the inset in TxPIC 10in FIG. 1. As previously indicated, the intensity modulation waveformimpressed on the channel signal for tagging could alternatively be asine wave instead of a square wave. The peak-to-peak value of themodulating square wave, V_(PP), together with the transfer function ofthe front PIN, determines the optical modulation index, which is madeconstant. While adjusting channel output power, the maximum and minimumvalues of V_(PP), i.e., V_(High) and V_(Low) would change to adjustaverage attenuation of the front PIN while maintaining opticalmodulation index constant. Depending on the output power setpoints forindividual channels, different signal channels may be controlled toproduce equal output powers or, if desired, unequal output powers, asused, for example, in channel pre-emphasis. This square wave modulationon each front PIN 18 is produced by generators 50 which will bediscussed in more detail later. Thus, a square wave intensity modulationwith a different square wave frequency is provided to each front PIN 18,and all channels on the TxPIC are simultaneously and synchronouslymodulated at all times. Each channel on the TxPIC is labeled by a uniquesquare wave, and the set of all square waves is chosen to beapproximately orthogonal over a selected integration interval.

In the embodiment of FIG. 1, lasers 14 are driven by a constant currentover life so that the combination of bias current, heater current, andsub-mount temperature is set at the time of manufacture to make theoutput wavelengths substantially close to a standardized grid desiredfor their respective wavelength emission operations, and to provideadequate output power for each channel. A laser heater 13 is providedlocally to each laser 14. Heater 13 can alter the optical carrierfrequency of each laser 14 in response to a change in joule heatingprovided via heater current which changes the temperature of laser 14 tochange its emission wavelength by a small incremental amount. Forexample, a TxPIC laser may tune with temperature at a rate ofapproximately −10.6 GHz/° C. Additional information in regard to suchheaters is set forth in nonprovisional applications, Ser. Nos.10/267,330 and 10/267,331, supra.

For bi-directional control of laser frequency employing such a heater13, laser 14 must operate at a locally elevated temperature so thatreduction of input power to the heater causes the laser temperature todrop, decreasing the laser emission wavelength. When the laser isoperated at an emission wavelength close to its standardized gridwavelength, then the accompanying laser heater 13 must supply nonzeropower. The cooling rate depends on the heater-supplied elevation inlaser temperature. The laser itself is then operated at a locallyelevated temperature so that in the case where it is desired to lowerthe laser emission wavelength, the current to the heater is reduced.Tradeoffs must be made between the cooling rate, long term aging budgetfor the laser wavelength shift, and thermal constraints for TxPIC 10. Itshould be realized that the minimal temperature elevation to be providedby a laser local heater at the end of its bidirectional control range isa small amount, from about 1° C. to about 2° C.

As previously indicated, optical intensity modulation, preferably asquare wave, is superimposed via the front PINs 18 in each of the Nsignal channels of TxPIC 10. The square wave modulation or tonefrequency is different for each channel and the modulating square wavevoltage is superimposed on front PIN 18 for each signal channel. Theamplitude of the tone drive voltage is contingent upon the requiredattenuation to be induced by the front PIN 18 and the local shape of thefront PIN transfer function for that desired attenuation setting. Thepeak-to-peak voltage of the modulating square wave is scaled to providea predetermined, constant optical modulation index (OMI) for thetone-modulated output light The aggregate optical channel group (OCG)from TxPIC multiplexer 20 is tapped at 34 and provided to theFabry-Perot wavelength locker (FPWL) sub-module 30. The tapped output issplit by splitter 31 between reference line 31A and etalon line 31B. Inetalon line 31B, the split light is provided to etalon 34. The referenceoptical signal on line 31A and etalon optical signal on line 31B aredetected by respective photodetectors PD1 32 and PD2 33 and theirrespective photo-detected current signals of PD1 and PD2 are convertedinto voltage signals by transimpedance amplifiers (TIA) 34A and 34B andare digitized by a fast analog to digital converter (ADC). The digitizedoutput waveform is correlated (multiplied by a delayed duplicate of themodulating square wave and integrated over a predetermined timeinterval) via a first field programmable gate array chip (FPGA1) 35.Note that PDs 32 and 33 function as low pass filters and only detect lowfrequency signals including the channel labeling tones. These PDstherefore do not respond to the high frequency data modulation such as10 Gbit/sec. or 40 Gbit/sec data modulation. In the signal processingassociated with FGPA1 35, the analog current signals are sampled over apredetermined time interval, such as 1 millisecond. Both output signals31A and 31B are both employed for the purpose of wavelength lockingwhere detected wavelength offsets are determined from these signals andlaser emission wavelength is accordingly adjusted via laser heaters 13.For the purpose of power control, however, only the reference output 3 1is needed for this purpose.

Front PIN 18 is, thus, the active transmissive channel element employedfor both a channel power control element and a modulation element forchannel labeling or identification. In the case of channel powercontrol, the front PIN provides for voltage-controlled attenuation overthe channel modulated signal to maintain it at a desired power level. Ininitial operation upon PIC manufacture, the individual channelattenuations are set for beginning of life (BOL) values selected toallow bidirectional control of channel power over life For example, thesource laser may exhibit power decay over life. Such decay over life maybe in the range, for example, from about 1 dB to about 3 dB, usuallyaround 2 dB. Also, as previously indicated, the front PIN averagereverse bias voltage is set by generator 50, preferably a square wavesource at a channel-specific frequency of a channel labeling tone. Tomaintain compatibility with per-channel power measurements, the opticalmodulation index (OMI) associated with a channel labeling tone is heldconstant at each channel as the average attenuation at element 18 isvaried over time. The OMI for the square wave is shown in FIG. 2. Thepeak to peak voltage swing of the modulating voltage, therefore, needsto change depending on the local slope of the transfer function, suchthat the OMI remains constant. The shape of the transfer functiondepends on how much optical power is being dissipated in the front PIN18 and will change over life of TxPIC 10 due to changes in output powerfrom laser 14 as well as other optical changes of other in-channeloptical elements upstream from front PIN 18.

While a sine wave modulation for tone applications may be generallyemployed in the art because orthogonality over an integration integralis more simply achieved, we employ square wave modulation because thatmodulation is easier to implement by simple switching between to voltagelevels at a desired frequency. In general, channel 1 on TxPIC 10 mayhave the lowest tone frequency for the optical channel group (OCG) andeach succeeding channel has a higher tone frequency separated by one ormore kHz. For example, the range of frequencies may be in the range ofabout 42.057 kHz to about 87.771 kHz spread over, for example, tensignal channels 15. However, the range of tone separation and channeldirection of tone increase can be in any order or direction. The kHztones may be of random value along the plurality of signal channels aslong as they are each at a different frequency and are chosen tomaintain proper orthogonality over the integration interval. The OMI isdefined as μ, which is the difference between the maximum output powerand the minimum output power over the sum of the maximum output powerand the minimum output power. In the case here, the OMI is shown as 5%,i.e., p=0.05, which corresponds to about 0.44 dB difference in thetransmitted powers between the modulation high point and the modulationlow point. The value for μ is simply the amplitude of modulation whenthe OMI is normalized to the average transmitted power, i.e., theaverage power after modulation from front PIN 18.

While in this description, the channel front PIN 18 provides for theabove described dual control functionality, it is also within the scopeof this disclosure as other embodiments to place the tone intensitymodulation alternatively on the semiconductor laser 14 in each channel15 or on the electro-optical modulator (EOM) 16 in each channel 15.Further, in another embodiment, it is possible to superimpose thechannel-labeling modulation on the channel modulator 16 of each channel15 primarily for wavelength tagging and control and employ the channelfront PIN 18 primarily for power control. Still in a further embodiment,a front PIN or similar optical element acting as a variable opticalattenuator (VOA) may be replaced with a semiconductor optical amplifier(SOA) in each channel 15 or a combination VOA/SOA may be placed withineach channel 15. Lastly, in a still another embodiment, the channelsignal tagging and wavelocking modulation may be superimposed on eachchannel modulator 16 with a channel SOA employed for providing powercontrol to achieve power equalization across the channel signal array.

1. Locking Wavelengths of a Multi-Channel Integrated TxPIC to aStandardized Wavelength Grid Values Using a Shared Wavelength ReferenceValue A. Wavelength Locking

A frequency tone, usually of low frequency, is placed on each lasercarrier frequency that functions as a different identification (ID) tagfor each laser as well as a means by which modulated source wavelengthcan be determined in an acceptable time period. Thus, the differentfrequency tones superimposed on front PINs 18 permit detection of eachoptical carrier frequency of each modulated source using a singlewavelength reference. These tone frequencies have frequencies well belowthe established bandwidth of wavelengths used to modulate data on eachof the channels, which is greater than 1 Gbits/sec. A discussion aboutthe use of tone frequencies in this manner is disclosed in some detailin U.S. patent application, Ser. No. 10/267,330, supra.

In FIG. 1, the Fabry-Perot wavelength locker (FPWL) sub-module 30provides the optical frequency reference values to which PIC lasers 14may be locked. For sufficiently high accuracy to be achieved, such as,with accuracy to the ITU grid around ±1.25 GHz, the servo loop 41, asrepresented by sub-modules 30 and 40, including two field programmablegate array (FPGA1 and FPGA2) chips 35 and 42, are supplied with acalibrated value of the ratio of the photocurrents supplied,respectively, from the etalon path 33A from etalon PD2 33 and itsaccompanying TIA circuit 34A and from the reference path 32A fromreference PD1 32 and its accompanying TIA circuit 34B at the desiredstandardized wavelength channel value. Also, the steady state errorservo loop must be zero. In the embodiment here, sub-module 30 isemployed, therefore, to provide a simultaneous reference for N TxPICchannels having a predetermined desired carrier frequency separation,such as in the embodiment here which is 200 GHz. Since there is adeployment here of a single FPWL 30 as a simultaneous reference for NTxPIC lasers 14 or signal channels 15, which are separated by a givenuniform frequency interval, e.g. 200 GHz, the use of just DCphotocurrents provided by the etalon and reference photodiodes will not,itself, suffice. This is why each channel signal on channels 15 issquare wave intensity modulated via its channel front PIN 18 at asignature fundamental tone frequency with all optical carriedfrequencies from the lasers so modulated continuously in parallel eachwith its own signature frequency, i.e., the frequencies of the tones aredifferent from one another.

Two field programmable gate array (FPGA1 and FPGA2) chips 35 and 42 areutilized in the feedback loop 41. FPGA2 42 has its own tone generatorfor the N tone frequencies, f_(k), for application to each of the Nsquare wave generators at 50 for application to each of the N front PINs18. FPGA 1 35 has its own square wave frequency generator to generatethe N tone frequencies, f_(k), for purpose of demodulation of the outputsignals from the FPWL photodetection circuits. Both photocurrents fromPD1 32 and PD2 33 are used for this purpose. A synchronization pulsefrom the tone generator at FPGA1 35 is provided to the tone generator atFPGA2 42 to maintain timing between the modulating tone and thedemodulating tones.

Square wave generators 50, as shown in FIG. 1, each comprise an analogswitch 51 that is driven at tone frequency, f_(k), by a voltage asreceived from FPGA2 42 via lines 49. Summer 52 is set with a negativebias via DACs 48 so that the switch 51 is modulated to be turned off andon according to the predetermined frequency for each respective channel.Thus, the DACs associated with a square wave generator set the highlevel and low level signals (V_(High) and V_(Low)), and the analogswitched is toggled at the frequency sent to it by FPGA2 42 V_(High) isprovided from DACs 48 via lines 48 to summers 52 from which the depth ofmodulation is determined by V_(Low) as illustrated graphically in thesquare modulation inset shown on TxPIC 10.

Error signals, representative of a particular laser wavelength offsetfrom its desired emission wavelength on the grid, are derived from thedigital signal processor (DSP) 44 to provide corrective changes ofcurrent to respective laser heaters 13 on TxPIC 10 via digital-to-analogconverters (DACs) 48 and lines 60 to the respective heaters 13. Digitalsignal processor (DSP) 44 is provided measurements of the tone strengths(for each channel) as detected by parallel demodulation of the outputsof the reference and etalon photodetector circuits. DSP 44 derives anappropriate error signal by, for example, taking the ratio of the tonestrengths for a particular channel, and comparing that ratio to acalibrated value corresponding to the desired wavelength. Also, DACs 48provide the set bias current, IL, to respective lasers 14 via lines 62from DACs 48. Also, DSP 44 receives back PD 12 photocurrents, VPD, fromeach channel for N lasers 14 on lines 64 via their respective TIAs 63and analog-to-digital converters (ADCs) 46. Thus, the monitoring andcontrol of heater currents and laser bias currents is accomplished byDSP 44 to prevent any attempt to drive lasers 14 to improper lockpoints.

In FIG. 1, the frequencies and phases of the N square waves provided bygenerators 50 are chosen to be approximately mathematically orthogonalover a fixed sampling period, such as, for example, approximately 1 ms.These square wave signals are synchronized at the start of each samplingperiod. From the basic set of square waves with an integer number ofcycles in a fixed sampling period, a nearly orthogonal set of N squarewaves can be selected, i.e., the product of two different square waveswhen integrated over a given interval, will provide a number very closeto zero, while the product of two identical square waves, whenintegrated over the same interval, will produce a large number that canbe normalized to unity. It is these normalized values that areindicative of channel wavelength drift via outputs derived from both theetalon output at 31B and the reference output at 31A and tone strengthderived from the reference output at 31A. The modulation tonefrequencies are chosen to be in a comparatively higher kHz range offrequencies to enable faster measurement of any wavelength shifts, e.g.,the tone frequencies are chosen to be greater than 40 kHz.

As indicated previously, the peak-to-peak voltage swing of eachmodulating square wave modulation supplied to the channel's front PIN,which is the difference between its high point or V_(High) and its lowpoint or V_(Low), is determined such as to provide a predetermined andconstantly maintained optical modulation index (OMI) in the output powerof each N signal channel. The percentage of OMI must be large enough toprovide an adequate signal to noise ratio for the signal processing andsmall enough to avoid excessive penalties for the data (e.g. due to eyeclosure. If the OMI is too small, circuit measurements will become tooinaccurate owing to circuit voltage offsets, dark current and leakagecurrent. If the OMI cannot is too large, the transmitted data willsuffer from eye closure penalties. As an example, a 5% OMI is anacceptable choice that induces a tolerable eye closure penalty whilestill producing large enough control signal to achieve a robust resultagainst signal errors and noise. The channel-specific voltage values forthe square wave required to produce such an OMI depends on the set DCbias point of each front PIN 18. Because their DC bias pointdeliberately varies as part of the per-channel power control, thevoltage values of the square wave modulation must be controlled by thedeployment of a lookup table or by the servo control loop 41 of FIG. 1to continually maintain a fixed OMI over time for all modulated tonefrequencies, f_(k).

It should be noted that etalon 34 can be procured with a specificperiodic response that matches the signal channel spacing or interval.However, we have chosen etalon 34 to have a periodic response of 50 GHzand its fringe transmission relative to signal channels k and k+1 isillustrated in FIG. 3. The reference signal in output 32A has a responsethat is approximately independent of optical wavelength. As previouslyindicated, the signal channel interval in the embodiment here is 200 GHzso that the etalon response will be a divisor of that spacing, i.e.,there will be three unutilized etalon fringes between the etalon fringesassociated with signal channels k and k+1. The same 50 GHz etalon is,therefore, useful to perform the same frequency discriminator functionfor other TxPIC channel spacings at integer multiples of the freespectral range, such as 50 GHz or 100 GHz, as well as for TxPICs havinga larger number N of signal channels, such as, for example, 40 channelsper TxPIC. Etalon 34 is a commercially available Fabry-Perot etalon thathas a 50 GHz periodic function versus optical frequency and is readilyavailable from different manufacturers, such as from JDSU, Inc. Themanufacturing process for the Fabry-Perot wavelength locker includessetting the relationship between the ITU grid and the transmission peaksof the etalon such that the ITU frequencies correspond to points roughlyhalfway up the sloped side of a transmission peak. The slope side of thetransmission peak, thus, acts as a local frequency discriminator wheresmall changes in optical carrier frequency produce detectable changes intransmitted power.

B. Locking to Correct Lockpoints

It should be noted that the higher frequency data modulation imposed onthe channel signals by the modulated sources 14, 16 is not within thebandwidth of the two photodiodes 32 and 33 in FPWL sub-module 30 and,therefore, does not directly appear in their photocurrent signals.Reference level 36 in FIG. 3 provides a point along the rising side(where optical power transmission increases as optical frequencyincreases) of an etalon fringe, where a useful lockpoint may beestablished. This point can be almost anywhere along the side of afringe, provided that the local slope is sufficiently large. Verticalreference level lines 37 that cross the fringes are used to indicate apoint of intersection between the ITU optical carrier frequency and theside of an etalon fringe, for a given sensed temperature of etalon 34.Sensor 39 is employed to passively determine where the point ofintersection is between a fringe in FIG. 3 with vertical reference 37and this accomplished by an initial calibration as to a set oftemperatures measured at etalon 34 over a range of possible temperaturesas related to a set of photocurrents for each PD 32 and 33 so that withthe use of this calibration, the relative values of the outputphotocurrents from PD 32 and PD33 are known at ITU grid frequencies.Thus, an established lockpoint 38 along the fringe side provides a meansby which detected values along the fringe side either up or downrelative to lockpoint 48 can be detected electronically. The amount ofoffset from calibrated lockpoint 38 is then employed to determine whichdirection and, when converted into digital form, by how much the laseremission frequency or wavelength has shifted from its desiredstandardized grid frequency. Thus, in the case of a single CW laser 14in a single optical channel 15, as opposed to multiple channels on aPIC, would produce the calibrated lockpoint ratio of DC photocurrentsfrom etalon and reference photodiodes 32 and 33 if the cw opticalchannel were at the lockpoint optical frequency on the standardizedgrid, such as the ITU grid.

When the tapped portion of the optical multiplexed signal from amultichannel TxPIC is supplied to the FPWL sub-module 30 and to etalonand reference photocurrent outputs 32A and 33A and corresponding TIAs34A and 34B, respectively, the resulting electrical signals will containcomposite signals, i.e., average DC values and tone modulations imposedon the combined channel signals as well as data modulation on thechannel signals, and noise, as well as from each of the N channels 15 onTxPIC 10. In the case of multi-channel input, an error signal for thek^(TH) optical carrier frequency is determined using coherentdemodulation to provide lock-in detection of square wave signalstrengths at frequency, f_(k), for etalon photodiode 33 and referencephotodiode 32. Since etalon 34 is sensitive to temperature changes, atemperature sensor 39, shown here as thermistor 39, is provided on FPWLsub-module 30 which allows for more precise calibration of the FPWLlockpoints to frequencies on the standardized grid, such as an ITU grid,as a function of the temperature of FPWL sub-module 30.

An error signal representative of the deviation of the mean opticalcarrier frequency of the k^(TH) laser 14 from its desired opticalcarrier frequency is extracted from the composite photocurrents fromboth TIA 34A and TIA 34B by coherently demodulating the tone signalstrengths from the respective PDs 32 and 33 at tone frequency f_(k) andcomparing the result to a calibrated value determined for the desiredoptical carrier frequency. As previously indicated, demodulating tonesare generated within FPGA1 35 and synchronized modulating tones aregenerated by FPGA2 at 42. For example, FPGA 35 may have a crystal clockto generate N tone frequencies for normalizing against the selected tonefrequencies from sub-module 30. Also, FPGA 42 generates the same set oftone frequencies which are synchronized with the tone frequenciesgenerated at FPGA 35 via a synchronization pulse sent from FPGA 35 toFPGA 42. The analog to digital conversion and coherent demodulation ofthe sample signal against the channel labeling tone are carried out byvia FPGA 35 and associated signal processing error signals are thenderived at DSP 44 which are filtered and scaled to provide forcorrective changes of current provided to individual laser heaters 13 atTxPIC 10. In this manner, the mean optical carrier frequencies of Nlasers are all driven to their assigned standardized grid frequencies orwavelengths, such as wavelengths assigned to the ITU grid, withsteady-state error equal to zero. Monitoring of TxPIC parameters,including but not limited to, heater currents and laser bias currents,are employed to prevent the feedback servo loop from attempting to drivethe TxPIC lasers 14 to improper lockpoints as more specifically pointedout later in this disclosure.

As previously indicated above, FIG. 3 illustrates a portion of thetransmittance of etalon 34 where a reference level 36 is shown at crosspoints with vertical reference lines 37 for two channels k and k+1defining lockpoints 38 for these channels. As indicated, the periodicspacing of etalon 34 here is 50 GHz. The TxPIC signal channel spacing is200 GHz grid so that adjacent TxPIC channels will be locked to fringesseparated by three intervening fringes or 150 GHz as seen in FIG. 3.Thus, for a given TxPIC laser 14 with an optical carrier frequency closeto a lockpoint 38, i.e., between next closest points of intersectionbetween reference line 36 and the side fringe of the periodic fringes, alaser frequency below or above a lockpoint 38 provides a difference oftransmissions with a properly assigned error as negative or positiveover a frequency range between lockpoint 38 and adjacent points alongthe channel side fringe. So, for example, if there is a positivefrequency offset in a given demodulated signal, the demodulated tonelevel will be above reference level 36. By the same token, if there is anegative frequency offset in a given demodulated signal, the demodulatedtone level will be below reference level 36. Thus, in the case of eachdemodulated tone, there is a window within which the demodulated signalvalue can be either above or below reference level 36 indicating inwhich direction, positive or negative, a correction must be made to movethe emission wavelength of a particular laser 14 through temperaturechange via its local heater 13.

The output of FPGA1 35 in the form of demodulated tone signals isprovided to DSP sub-module 40 and to DSP 44 via line 41. The output ofFPGA2 42 is provided to digital signal processor (DSP) 44. Sub-module 40includes associated digital-to-analog converters (DACs) 48 andanalog-to-digital converters (ADCs) 46.

The signal processing that produces a demodulated tone signal is asfollows. The output photocurrent from PD1 at 32 and PD2 at 33 isconverted to a voltage at TIA 34A and 34B. The voltage level of the TIAsmust be scaled appropriately relative to the input range allowed at afast analog to digital converter. In FPGA1 at 35, the digital product ofa bipolar square wave (allowed values +1, −1, with fundamental periodequal to that of the particular channel labeling frequency) and thesampled waveform is formed, and an accumulation product equivalent tointegration of the product of the bipolar square wave and the sampledwaveform is calculated. The interval for integration contains an integernumber of cycles of each of the channel-labeling square waves, and thechannel-labeling square waves were chosen to be approximatelyorthogonal. Thus, the output of this FPGA integration at FPGA1 at 35 isa large number proportional to the strength of the signal modulated bythe selected square wave. This, in turn, is proportional to the productof the optical modulation index (OMI) and the average optical power ofthe channel labeled by the selected square wave, at the receivingphotodiode. For etalon channel 31B, the average optical power of thechannel depends on the optical carrier frequency of the channel, becausethe etalon fringe serves as an optical frequency discriminator. Forreference channel 31A, the average optical power of the channel isindependent of optical carrier frequency. Thus, FPGA1 35 digitizes thetone frequencies for both voltage signals from the etalon input and thereference input. Parallel demodulation (via correlation with theapproximately orthogonal square waves) for each of the N channelsprovides signals in the form of a measure of the respected tonestrengths for each channel 15 of TxPIC 10. These signals are sent to DSP44, via line 41, where an error signal for each channel is calculated.Further, DSP 44 then provides correction values, via DACs 48 and lines60, in the form of correction signals to individual, respective laserchannel heaters 13 of lasers 14 based upon calculated error signalsdetermined in parallel for each signal channel. Each such correctionsignal changes the operating temperature of the laser via its associatedheater which, in turn, will change the emission wavelength of the laserto correct that emission wavelength to be closer to or at itsgrid-designated and desired emission wavelength. In another embodiment,rather than change the current applied to a laser heater 13, thefeedback loop 41 can be employed to change the bias current of a laser14 in order to correct that emission wavelength to be closer to or atits grid-designated and desired emission wavelength. In the embodimenthere, however, it is preferred that lasers 14 are operated at a constantcurrent bias value over life and that each laser is provided with anassociated on-chip heater 13 that changes the laser operatingtemperature which, in turn, changes its emission wavelength in adirection toward or to its standardized wavelength grid frequency.

As previously indicated, the wavelength locker operation at sub-module30 can vary with ambient temperature so that an ideal set point for eachemission wavelength for N lasers is initially calibrated duringmanufacturing for a whole range of operating temperatures and thesecalibrated setpoints are stored in a memory of DSP 44. Thus, at a givenoperating temperature of etalon sub-module 30, which is monitored bytemperature sensor 39, the setpoint can be interpolated linearly betweencalibrated values for two adjacent temperature calibration points in theDSP memory. The error signal employed for wavelength locking feedback isbased upon the difference between the calibrated setpoint and theinterpolated setpoint. It can be seen that this approach provides anestimate of the wavelength for each tone modulated signal at givencalibrated thermal setpoints for sub-module 30 so that the interpolationwill provide a fairly accurate estimation of the wavelength offset, ifany, of each laser 14 from its standardized grid wavelength.

As indicated above, the outputs of both the reference and etalonphotodiodes 32, 33 are simultaneously demodulated for all N signalchannels on TxPIC 10. A bi-linear combination of coherently demodulatedphotocurrents at a particular tone frequency from these photodiodes isemployed to define a suitable error signal: $\begin{matrix}{{ErrorSignal} = \frac{\left( {I_{Etalon} - {{\kappa(T)}I_{{REF}\quad}}} \right)}{\left( {I_{Etalon} + {{\kappa(T)}I_{{REF}\quad}}} \right)}} & (1)\end{matrix}$

where κ(T)=I_(Etalon)/I_(REF) is evaluated at the standardized channelfrequency to which the optical carrier frequency for the given signalchannel should be locked to at a given temperature, T, as retrieved fromtemperature sensor 39 on FPWL sub-module 30. FIG. 3 illustrates anidealized case in which the etalon photocurrent is equal to thereference photocurrent, i.e., κ(T)=1, for all channel frequenciesseparated by 50 GHz. For a given optical channel and a given temperaturefor the Fabry-Perot wavelength locker, the ratio of the demodulatedphotocurrents corresponding to the desired optical carrier frequency(e.g. ITU grid frequency) is known from calibration. The differencebetween this calibrated value and the measured value provides a signedquantity indicative of the deviation of the channel optical carrierfrequency with respect to the desired optical carrier frequency. Oncethe error signal is measured, the corrective actions to be taken by thechannel's heater can be calculated and applied. One form of an errorsignal for signal channel, k, is formed in DSP 44 from the combination,$\begin{matrix}{{Error} = {{\frac{\left( {I_{ET} - I_{{REF}\quad}} \right)}{\left( {I_{ET} + I_{{REF}\quad}} \right)}\quad{Measured}} - {\frac{\left( {I_{ET} - I_{{REF}\quad}} \right)}{\left( {I_{ET} + I_{{REF}\quad}} \right)}\quad{Calibrated}}}} & (2)\end{matrix}$

where I_(ET) is the demodulated etalon photocurrent signal for channel kand I_(REF) is the demodulated reference photocurrent signal for channelk. “Measured” in formula (2) refers to the result of the demodulationprocess and, “calibrated” refers to a previously measured or calculatedvalue determine by the calibration process used to define the responseof the etalon and reference paths to an optical carrier frequency equalto the desired value. The temperature of the Fabry-Perot wavelengthlocker might be temperature-controlled, in which case a singlecalibration value can be used. Alternatively, a Fabry-Perot wavelengthlocker without temperature control can be calibrated at a variety oftemperatures, and interpolation can be used to establish the appropriatecalibration value at a given measured temperature for the wavelengthlocker. The error signal is the input to a feedback loop that drives theerror signal to zero. The loop-filtering of the error signal isdigitally implemented by DSP 44 and provided to the digital to analogconverter (DAC) to produce a correction signal that is a scaled analogoutput signal that controls the current to the respective heater 13 forthe laser in channel, k, to drive the error signal to zero underintegral control.

Of course, FPWL manufacturing tolerances cause deviations from the idealfeedback loop condition so that is why it is important that measurementsbe calibrated relative to the temperature of FPWL sub-module 30 asmonitored by temperature sensor 39. The feedback loop 41 of FIG. 1 isdesigned to drive the optical carrier frequency of a laser in thedirection opposite to the algebraic sign of the error signal. In FIG. 3,the error signal has three possible adjacent zero-crossings in thevicinity of each local fringe. For the control loop to lock the opticalcarrier frequency to the correct local setpoint when the feedback loop41 is engaged, the laser optical carrier frequency must lie within agiven range around the desired setpoint. In order to make sure that theloop locks the optical carrier frequency to the correct fringe among theperiodic fringes of the Fabry-Perot etalon, a lookup table based oncalibration of laser currents, heater currents, submount temperatures,and interactions among multiple channels of a TxPIC may be provided.Given the appropriate calibration information and measured values forall of these frequency-affecting inputs, DSP 44 can determine whetherthe range of applied values correspond to a valid optical frequencysetting affiliated with a particular fringe of the Fabry-Perotwavelength locker. This is discussed further below in connection withrapid response to channel failure conditions.

DSP 44 includes an integrator that integrates the error signal for eachgiven laser wavelength over time (after scaling by affixed gainconstant) to achieve an average value which is provided to a particularDAC 48 which, in turn, generates an analog correction signal that drivesa particular heater 13 for a particular laser 14. If the integratoroutput is a positive correction signal, this corresponds to a red shift(lower frequency or wavelength) and an increase in heater power. On theother hand, if the integrator output is negative correction signal, thiscorresponds to a blue shift (higher frequency or wavelength) and adecrease in heater power. The integrator saturates such that, for afixed frequency offset, a correction signal to the given laser willalways end up with the correct polarity. If for any reason, thecorrection signal exceeds a predetermined laser adjustment range, suchas, for example, ±1 to 5 GHz, then the wavelength locker loop controllerwill shut down the operation of the particular PIC laser affected. Thisis an indication that the laser is not properly operating and needs tobe taken out of service.

In connection with the foregoing, FIG. 5 shows graphically illustrates ared shift relative PD2 33 current, iPD versus optical frequency relativeto an etalon fringe as i_(PD-Etalon). The point A at i_(PD-Ref) is thedesired lockpoint where the grid wavelength or frequency is proper for astandardized (ITU) grid frequency for a given modulated source outputwavelength. The red shift at i_(PD-Shift) to point B is the amount ofdecrease from iPD-Ref indicative (proportional) to the amount ofdecrease in emission wavelength of a laser 14 from the standardized(ITU) grid frequency. This shifted amount of error can be expressed asthe following ratio: $\begin{matrix}{{Error} = \frac{i_{{PD} - {Etalon}}}{i_{{PD} - {Ref}}}} & (3)\end{matrix}$

A correction signal is developed to blue shift the wavelength operationat laser 14 via heater 13, i.e., increase applied heat to the laser, sothat the wavelength operation of the laser is brought back from point Bto be as close as possible to point A.

It should be noted that when the operation of the wavelength lockersystem of FIG. 1 is first turned on or in the case of a fringecorrection event (triggered by detection of an invalid setting at DSP44), the laser heater outputs are preset to an expected value and to anegative frequency offset (red shift) so as to allow for thermalstabilization prior to operation of the feedback wavelength control loop41. Laser heater power as well as the derived heater resistance iscontinuously monitored by the wavelength locker loop DSP 44, via theADCs 48, and the heater drive circuits (not shown). If the derivedresistance is outside a predetermined threshold range, then, wavelengthlocker loop controller DSP 44, will drop this channel laser fromoperation. Of course, during startup, the heater power is allowed toinitially stabilize before any such action is taken by the wavelengthlocker loop DSP 44.

C. Prevention of Locking to Incorrect Lockpoints

The feedback wavelength control loop 41 illustrated in FIG. 1 withoutother information would not be able to determine if it had locked agiven laser carrier frequency to an incorrect lockpoint, that is, itmay, instead, have locked the frequency to an adjacent incorrect fringerather than the correct fringe. In other words, since the wavelengthlocker characteristic is periodic, the derived error signal, previouslydiscussed, provides no indication in and of itself of the particularfringe in FIG. 3 to which a particular optical carrier frequency islocked, yet the tuning range for each laser in the PIC encompassesmultiple fringes. Also, thermal coupling on TxPIC chip 10 betweenintegrated elements on the chip as well as to a thermoelectric cooler(TEC) (not shown), underlying the TxPIC chip 10, may be sufficientlystrong that (1) a thermal transient may cause a particular laser'sfrequency to jump to align to an incorrect fringe before the wavelengthfeedback loop 41 could correct for such a thermal transient; (2) thermaltransients generated by the disabling of, or the failure of, anotheradjacent laser or laser heater on the chip are of adequate magnitude tocause a jump to an adjacent fringe; (3) at initialization and poweringup of TxPIC 10, the last known good laser heater values will notreliably initialize the laser frequency or wavelength to within adesired capture range if a change has been made to the enabled ordisabled state of any of the PIC lasers 14 or their heaters 13; or (4)thermal changes due to normal laser aging vary thermal crosstalk intoeach adjacent laser and this crosstalk effect is accounted for byperiodically storing of the last know good heater values which maydifferent due to different laser aging. Thus, additional informationmust be employed to prevent the wavelength control loop 41 frominadvertently driving a laser wavelength toward another incorrectfringe, resulting in controlling the optical carrier frequency to anincorrect setpoint.

This additional information is provided in a lookup table in the memoryof DSP 44 that includes all of the last known good values for alloperating parameters of the TxPIC 10 that particularly affect the lasercarrier frequency. The predominant set of these operating parameters fora given TxPIC are: (1) the laser bias currents, which, it will berecalled, are held fixed throughout TxPIC life; (2) heater currents,which, it will be recalled, are varied by feedback control throughoutTxPIC life; (3) the (fixed) temperature of the TxPIC submount (notshown), which is generally mounted on a thermoelectric cooler (TEC) (notshown); (4) the electro-optic modulator (EOM) bias voltages; and (5) thefront PIN reverse bias voltages. Also, as previously discussed above,there is a need for a lookup table of calibrated lockpoint ratios k(T)for all N signal channels on TxPIC 10. Lastly, there is a need forlookup tables of heater-induced tuning coefficients for all TxPIC Nchannels including their channel crosstalk coefficients. Using thisinformation, as stored in DSP memory, DSP 44 can determine from thisinformation in the lookup tables whether the feedback wavelength loop iscurrently being asked to provide heater driver powers that areinconsistent with last known good lockpoints. Note, as previouslyindicated, that adjacent available lockpoints are separated by the freespectral range (FSR) of the etalon, which here is 50 GHz. To move anindividual laser optical carrier frequency from a correct lockpoint inFIG. 3 to an incorrect adjacent lockpoint requires a laser heater toattempt to raise or lower its associated laser temperature by as much asabout 5° C., which is a large, easily-detected change in heater power.Thus, the current supplied to laser heaters 13 are monitored by DSP 44for comparison to the last known good values or average values ofapplied current to the respective laser heater 13 on TxPIC 10. In thiscase, DSP 44 will prevent such an application of improper laser heaterpower to be applied, particularly outside its range of power normalcybased on this last known good tracking methodology.

2. Per Channel Transmit Power Control Loop

Also, in this disclosure, is a method for determining the relativebalance of the channel powers which leverages the same channel-labelingmodulation described above for the wavelength locking control usingfeedback control loop 41. A per-channel power control loop operates toequalize channel power across the array of signal channels forming theoptical channel group (OCG). More generally, a per-channel power controlloop operates to hold individual optical channel powers at individualsetpoints that may differ from one channel to the another. When used toequalize channel powers, the function of the per-channel loop is toequalize channel power across the channel array to within apredetermined error, such as, for example, ±0.50 dB, for any channelpower relative to the average powers within the OCG over the life ofTxPIC 10. This power loop is initiated at turn-up during initializationof TxPIC 10. With reference again to FIG. 1, each channel power outputis controlled by its front PIN 18. Having the front PIN serve the dualpurpose of individual channel power control and channel-labelingmodulator reduces the number of active channel elements and controlelectronics required Thus, element 18 may also be referred to as amultifunction element (MFE) and can be referred to as anattenuator/modulator tone PIN front PIN 18 absorbs a portion of themodulated channel signal received from the channel modulated source 14,16 based on an average bias voltage determined by the combination of avariable bias voltage, V_(High), and a peak-to-peak voltage, V_(PP),supplied from DSP 44 in sub-module 40 via generator

The amount of attenuation required for each channel is determined by twofactors. First, the multiple laser optical outputs at beginning of life(BOL) are not perfectly balanced and may vary from one another by agiven amount. At manufacture, the optimum constant drive current foreach laser is determined such that: (1) each laser 14 can be wavelengthlocked within the acceptable limits of laser heater power; (2) the laserpower can be adjusted to within the required tolerance of its front PIN18; and (3) the desired bit error ratio in transmission links can be metwhen high frequency data modulation is applied to the channel modulator16. As previously indicated, per channel laser drive current is heldconstant over the life of TxPIC 10. The proper laser bias current valuesare initially determined during PIC module manufacturing and thepredetermined values for each laser are transferred and stored inmemory, such as a flash memory, in DSP 44 in sub-module 40. However,calibration of the laser drive currents on TxPIC 10 itself does not, inand of itself, insure power balance across the array of signal channeloutputs on the PIC so that a value of attenuation that will be requiredvia a front PIN 18 for each TxPIC channel to be set to achieve powerequality across the channel outputs, and these front PIN attenuationvalues will vary from channel to channel over PIC life.

Second, the integrated lasers 13 in TxPIC 10 age at different rates overthe life of the circuit and, normally, throughout this aging process,their output powers decrease, again, at different rates. Therefore,front PIN attenuation for each signal channel must typically begradually reduced as the laser power decays; i.e., the average reversebias voltage on front Pin 18 typically has to be decreased over life.Therefore, the applied negative bias is highest at the beginning of life(BOL) of PIC lasers 14 and is generally decreased over life to maintaina substantially constant power output from the modulated sources overPIC life. Thus, lasers 14 at BOL are operated at a continuous appliedcurrent level for life with a high magnitude of output power. Theattenuation is set for each laser at the desired initial power leveloutput from each channel and the power outputs across the channel arrayare substantially equalized. Since lasers 14 each age at a differentrate, which are not individually predictable, their adjusted outputpowers will decrease at different rates over life, and, therefore, it isnecessary that the attenuation be withdrawn, i.e., negative bias isreduced at front PINs 18, from each channel output over life at adifferent rate too. Similarly, increases of individual channel powerscan be compensated by increases in attenuation settings of front PINswhere necessary.

Therefore, power control feedback loop 41 is employed to determine theamount of attenuation per channel to apply in order to continuallymaintain a balance of channel power outputs across the array ofmodulated channel signals produced on TxPIC 10. The average bias pointfor each front PIN 18 is, therefore, calculated at DSP sub-module 40 viaDSP 44 and is applied to each front PIN 18 in a closed loop control 41along with the application of a tagging tone frequency to each front PIN18 for signal channel identification. The average bias voltage is themean of the voltages V_(High) and V_(Low), where values of V_(High) andV_(Low) are chosen to maintain a constant optical modulation index. Thepower control loop 41 operates on a relatively long time constant, suchas around 5 seconds, and is designed to adjust the attenuation levelbased upon the manufacturing variables, mentioned above, and laserdevice aging, which, of course, is a comparatively slower process thanwavelength changes from a desired wavelength. This time period couldalso be set for longer periods of times, if desired, since laser agingis, comparatively, a much slower process.

As previously indicated, a low frequency or tone amplitude modulation issuperimposed on the front PINs 18 in each of the N signal channels ofTxPIC 10. The values of the voltages applied are contingent upon theestimated attenuation curve for front PINs 18 and are scaled so as toprovide a particular optical modulation index (OMI), as previouslydiscussed. Also, as indicated earlier, the optical channel group (OCG)multiplexed signal from TxPIC multiplexer 20 is tapped and provided tothe Fabry-Perot wavelength locker sub-module 30 where the etalon outputon line 33A and reference output on line 32A are amplified and digitizedand are together employed for determining wavelength offset andadjustment. However, as noted, etalon output 31B is not used forpurposes of power control. Only the reference output 31A is necessaryfor this purpose. The digitized OCG envelope from the reference output31A is coherently demodulated at FPGA1 35 for each tone. This process issimultaneous, i.e., done in parallel for all N signal channels such thatdata is continuously provided for all channel tones. At sub-module 40 inDSP 44, the N channel demodulator outputs from FPGA1 35 via line 41 canthen be respectively compared with the originally generated modulatedtones to obtain the relative amplitude of each of the N tones for eachsignal channel 15. Given that the OMI is held at constant value, such asat 5% for each tone, the relative carrier amplitude detected for eachchannel is the same as the relative tone amplitude. Thus, this channelpower detection scheme assumes, the same OMI for each channel tonemodulation. As described previously, calibration of individual front PINtransfer functions is required in order to determine the correct valuesof the voltage rails of an applied square wave to provide the requiredattenuation while holding the optical modulation index constant. Also,the front PIN transfer function changes as the incident laser powerchanges. Therefore, the initial front PIN transfer function must becalibrated at the time of manufacture and the changed over life must beestimated since the laser incident power on front PIN 18 cannot readilybe directly measured.

Thus, the attenuation versus bias curve for each front PIN 18 atbeginning of life (BOL) is calibrated during initial manufacturing withthe laser operating at a fixed drive current, which current level ispredetermined during calibration by varying the bias over a givenvoltage range and measuring the output power from TxPIC 10. A pluralityof voltage steps are, therefore, taken through the voltage range atequal steps in bias voltage. This first set of output powers as afunction of applied reverse bias voltage on the front PIN is measuredexternally for each channel. The set of points (normalized attenuationversus reverse bias) is stored in the memory of DSP 44, together withthe corresponding Back PIN current that provides a measure of the outputpower of the laser. Then, a second set of a plurality of points(normalized attenuation versus reverse bias, with an associated Back PINcurrent) is taken and stored, but this time with the applied laser drivecurrent intentionally decreased for each laser to simulate an end oflife (EOL) power level across the array, and with all wavelengths heldconstant at their BOL values. This simulation is based upon agingexperience of the laser array on TxPIC 10 where, the decay of laserpower output may be in the range of about 1.5 dB to about 3 dB overlife.

More particularly, then, at manufacture, two normalized front PINtransfer functions are created; one for the beginning of life (BOL)state and one for the simulated end of life (EOL) state. An algorithm isemployed that interpolates between the two transfer functions, based onthe reading of the Back PIN current. The interpolated, normalizedtransfer functions are used to calculate appropriate values of V_(High)and V_(Low), as depicted in FIG. 4, for each signal channel to providethe desired attenuation while holding the optical modulation indexconstant.

Therefore, during operation, the real-time photocurrent received fromback PDs 12 is employed to estimate the laser power. Then, the beginningof life (BOL) and the simulated end of life (EOL) front PIN curves,relative to the two different mentioned sets of data above, are linearlyinterpolated to form a new front PIN attenuation curve associated withthe estimated laser power. Based upon the newly derived normalizedtransfer function, two voltages V_(High) and V_(Low) are chosen suchthat, (1) the desired demodulated tone signal is obtained at referenceoutput 31A in the FPWL sub-module 30 and (2) the estimated OMI ismaintained at a predetermined value, such as an OMI at 5%. For a givenchannel, the demodulated tone signal is proportional to the product ofthe optical modulation index and the average optical power of thechannel labeled by the channel-labeling tone used for demodulation. Ifall demodulated tone signals are made to be equal as seen the referencephotodiode in the Fabry-Perot wavelength locker, and if all channelshave the same optical modulation index for their (orthogonal) channellabeling tones, then all channel powers are approximately equal at thereference photodiode. This is the basis of optical channel powercontrol. Note that the reference photodiode in the Fabry-Perotwavelength locker serves dual roles as the detector for optical channelpower control and as one of two detectors needed for the wavelengthlocking control.

While the invention has been described in conjunction with severalspecific embodiments, it is evident to those skilled in the art thatmany further alternatives, modifications, and variations will beapparent in light of the foregoing description. Thus, the inventiondescribed herein is intended to embrace all such alternatives,modifications, applications and variations as may fall within the spiritand scope of the appended claims.

1. A multi-channel transmitter photonic integrated circuit (TxPIC)comprising: a plurality of integrated signal channels; an integratedmodulated source in each signal channel having a different emissionwavelength from other modulated sources and providing an opticalmodulated signal; an integrated active element in each signal channelthat is transmissive of the channel optical modulated signal and uponwhich is superimposed a modulated tone frequency that is employed todetermined any a channel emission wavelength and channel power offsetfrom a predetermined emission wavelength and power level for eachchannel modulated source.
 2. The multi-channel transmitter photonicintegrated circuit of claim 1 further comprising a local heater elementprovide for each modulated source to tune an emission wavelength of acorresponding modulated source to its predetermined emission wavelength.3. The multi-channel transmitter photonic integrated circuit of claim 1wherein the active element in each signal channel comprises a p-i-n(PIN) junction device, a laser, an optical modulator, a semiconductoroptical amplifier (SOA), a variable optical attenuator (VOA), aphotodetector (PD), or the modulated source itself.
 4. The multi-channeltransmitter photonic integrated circuit of claim 1 wherein the modulatedsource comprises a directly modulated laser or a continuous wave (cw)laser with an external electro-optic modulator.
 5. The multi-channeltransmitter photonic integrated circuit of claim 1 further comprising anintegrated multiplexer formed in the circuit that receives the channeloptical modulated signals from the active integrated element andcombines them into a WDM signal that is place on an output of thecircuit.
 6. A monolithic photonic integrated circuit (PIC) comprising: aplurality of signal channels integrated on a single substrate; amodulated source in each signal channel for producing an opticalmodulated channel signal of a predetermined channel emission wavelengthand channel power; a transmissive active element in each signal channelto receive the optical modulated channel signal; and a modulated tonefrequency applied to each transmissive active element, each such tonefrequency different from other respectively applied tone frequencies,from which is determined the amount of variation from the modulatedsource predetermined channel given emission wavelength and channelpower.
 7. The monolithic photonic integrated circuit (PIC) of claim 6further comprising a feedback control circuit to produce an error signalfor each signal channel, if any, from which is derived a correctionsignal to correct modulated channel emission wavelength and channelpower.
 8. The monolithic photonic integrated circuit (PIC) of claim 7wherein channel wavelength control of the optical modulated channelsignal is carried out by change to either modulated source applied biascurrent or operating temperature.
 9. The monolithic photonic integratedcircuit (PIC) of claim 8 further comprising a local heater in proximityto each modulated source, the correction signal applied to the heater tochange the emission wavelength of the modulated source.
 10. Themonolithic photonic integrated circuit (PIC) of claim 9 wherein thecorrection signals are applied toward reducing the error signal to zero.11. The monolithic photonic integrated circuit (PIC) of claim 7 whereinchannel power control is carried out by changing applied bias to theactive element.
 12. The monolithic photonic integrated circuit (PIC) ofclaim 6 wherein the modulated tone frequency applied to each channel isa square wave or a sine wave with an applied bias to control the levelof power output of the optical modulated channel signal from thetransmissive active element.
 13. The monolithic photonic integratedcircuit (PIC) of claim 6 further comprising an integrated opticalcombiner on the PIC that receives the optical modulated channel signalsfrom the signal channels and combines them into a WDM signal for outputfrom the circuit.
 14. The monolithic photonic integrated circuit (PIC)of claim 13 wherein the optical combiner is a wavelength selectivecombiner or a free space coupler combiner.
 15. The monolithic photonicintegrated circuit (PIC) of claim 13 wherein the optical combiner is anarrayed waveguide grating, an Echelle grating or a MMI coupler.
 16. Themonolithic photonic integrated circuit (PIC) of claim 6 wherein thetransmissive active element comprises a p-i-n (PIN) junction device, alaser, an optical modulator, a semiconductor optical amplifier (SOA), avariable optical attenuator (VOA), a photodetector (PD), or themodulated source itself.
 17. The monolithic photonic integrated circuit(PIC) of claim 6 wherein the modulated sources are directly modulatedlasers in each signal channel or continuous wave (cw) lasersrespectively coupled to an external electro-optical modulator in eachsignal channel.
 18. A monolithic photonic integrated circuit (PIC)comprising: a plurality of signal channels integrated on a singlesubstrate; a modulated source in each signal channel for producing anoptical channel signal of a given emission wavelength; a transmissiveactive element in each signal channel; a modulated tone frequencyapplied to the transmissive active element and modulates the opticalchannel signal that identifies each signal channel; an integratedoptical combiner in the PIC that combines the optical channel signals inthe PIC into a WDM signal provided as an output from the circuit; and afeedback controller that receives a portion of the WDM signal,demodulates the WDM signal portion into individual tone channel errorsignals indicative of wavelength drift of a modulated source emissionwavelength from a desired wavelength value and derives a correctionsignal to drive respective modulated source emission wavelengths towardor to their desired emission wavelength.
 19. The monolithic photonicintegrated circuit (PIC) of claim 18 wherein the error and correctionsignals for the modulated sources are derived in parallel for the signalchannels.
 20. The monolithic photonic integrated circuit (PIC) of claim18 wherein the transmissive active element includes an applied bias toregulated the level of output power in each signal channel to a desiredpower output level.
 21. The monolithic photonic integrated circuit (PIC)of claim 20 wherein an error signal is also derived in the feedbackcontroller for the WDM signal portion indicative of a variance ofchannel output power from a desired power level from which is derived apower correction signal to change the applied bias to the transmissiveactive element of the signal channel to change the output power levelper channel to the desired output level.
 22. The monolithic photonicintegrated circuit (PIC) of claim 20 wherein the output power level ofthe signal channels are render to be substantially equal as provided tothe optical combiner.
 23. The monolithic photonic integrated circuit(PIC) of claim 18 wherein the optical combiner is a wavelength selectivecombiner or a free space combiner.
 24. The monolithic photonicintegrated circuit (PIC) of claim 18 wherein the transmissive activeelement comprises a p-i-n (PIN) junction device, a laser, an opticalmodulator, a semiconductor optical amplifier (SOA), a variable opticalattenuator (VOA), a photodetector (PD), or the modulated source itself.25. A feedback system for a monolithic photonic integrated circuit (PIC)comprising: a plurality of integrated optical signal channels formed onthe PIC with a modulated source in at least some of the channels forprovided a plurality of modulated channel signals each having adifferent predetermined emission wavelength; an optical combiner thatreceives the modulated channel signals and combines them into a WDMsignal for output from the circuit; a feedback circuit coupled toreceive a portion of the WDM signal output to demodulate the WDM signaland determine from the respective demodulated channel signals whether ornot the emission wavelengths of the modulated sources in each channelare offset from the predetermined emission wavelength; the feedbackcircuit generating error signals in parallel for each signal channelrepresentative of the predetermined emission wavelength offsets for eachsignal channel and generating correction signals for applying emissionwavelength changes to the modulated sources; and a wavelengthcompensator associated with each modulated source to receive arespective correction signal to cause the modulated source emissionwavelength to be adjusted to or closer to its predetermined emissionwavelength.
 26. The feedback system of claim 25 further comprising anintegrated transmissive active element in each signal channel thatreceives a modulated tagging signal that modulates the channel signalpassing through the element, the frequency of the modulated tag signalsdifferent for each signal channel, and the range of frequencies of themodulated tag signals different from a range of frequencies of thechannel modulated sources.
 27. The feedback system of claim 26 whereinthe integrated transmissive active element is a p-i-n (PIN) junctiondevice, a laser, an optical modulator, a semiconductor optical amplifier(SOA), a variable optical attenuator (VOA), a photodetector (PD), or themodulated source itself.
 28. The feedback system of claim 26 wherein themodulated tag signals are tone-based frequencies below the frequencyrate the modulated sources.
 29. The feedback system of claim 25 whereineach modulated source comprises a modulated semiconductor laser or acontinuous wave semiconductor laser with an external modulator.
 30. Thefeedback system of claim 25 wherein the WDM signal output portionincludes applied tagging frequencies for each modulated channel signaleach with each tagging frequency different from any other taggingfrequency, and a circuit to demodulate the tagging frequencies which areemployed to determine any difference between the current emissionwavelength of each signal channel from its predetermined emissionwavelength.
 31. The feedback system of claim 30 wherein the appliedtagging frequencies are employed in the feedback circuit to determinechannel output power levels and correct the channel power levels to besubstantially the same across the signal channels.
 32. The feedbacksystem of claim 31 further comprising an integrated transmissive activeelement in signal channel to receive the power level correction thatmakes changes to its applied bias.
 33. The feedback system of claim 25further comprising a transmissive active element in each signal channel.34. The feedback system of claim 33 wherein the transmissive activeelement comprises a p-i-n (PIN) junction device, a laser, an opticalmodulator, a semiconductor optical amplifier (SOA), a variable opticalattenuator (VOA), a photodetector (PD), or the modulated source itself.35. The feedback system of claim 33 wherein a unique modulated tonefrequency is applied to each transmissive active element as a channelidentifier, the tone frequencies employed in the feedback circuit toprovide for adjustment to each signal channel emission wavelength andpower output.
 36. The feedback system of claim 25 further comprising thefeedback circuit generating correction signals for correcting channeloutput power to a predetermined power level in each channel and atransmissive active element in each channel having a bias that isadjusted by the correction signal so that the channel signal output fromthe active element is adjusted to or closer to the predetermined powerlevel.
 37. The feedback system of claim 36 wherein the predeterminedpower levels for all signal channels are made to be substantially equal.38. A method for correcting signal channel emission wavelength andchannel output power in a monolithic photonic integrated circuit (PIC)having a plurality of integrated signal channels, comprising the stepsof: generating a plurality of modulated channel signals in the circuit;tagging each modulated channel signal with a unique tone frequencysignal; combining the modulated channel signals into a WDM signal in thecircuit and providing the WDM signal at a circuit output; demodulatingthe tone frequency signals from a portion of the WDM signal output; andderiving error signals from demodulated tone frequency signalsindicative of channel changes in emission wavelength or channel outputpower; and deriving correction signals based upon the error signals forapplication to each signal channel to change channel emission wavelengthor channel output power to be as close as possible to a predeterminedemission wavelength and a predetermined power output level desired foreach integrated signal channel.
 39. The method of claim 38 wherein thestep of tagging is carried out by utilizing a transmissive activeelement in each signal channel to which the unique tone signal isapplied.
 40. The method of claim 39 comprising the further step ofbiasing the transmissive active element in each signal channel andcontrolling the applied bias level to control the channel output power.41. The method of claim 40 wherein the applied bias level in each signalchannel is controlled so that the channel output power across the signalchannels is substantially equal.
 42. The method of claim 38 comprisingthe further step of applying the correction signal to the modulatedsources of the signal channels to change their emission wavelength. 43.The method of claim 42 wherein the step of applying the correctionsignal to the modulated sources is carried out by changing applied biasto the channel modulated source or changing local temperature of themodulated source.
 44. A method of generating a plurality of opticalmodulated signals having predetermined operating characteristics from amulti-channel transmitter photonic integrated circuit (TxPIC),comprising the steps of: providing a modulated source in each signalchannel to produce a modulated signal having a predetermined desiredpower level and having a predetermined desired emission wavelength whichis different from the predetermined emission wavelength of the othersignal channels; providing an active element in each signal channel thatreceives the channel modulated signal; and applying a modulated tone tothe channel signal via the active element from which is derived bothrespective current channel emission wavelengths and power outputs of thechannel signals.
 45. The method of claim 44 comprising the further stepof changing the emission wavelength of respective signal channels basedupon information derived from the modulated tones.
 46. The method ofclaim 45 wherein the emission wavelength is changed by changing eitherthe bias current to the modulated source or changing the localtemperature of the modulated source.
 47. The method of claim 44 changingthe power output of respective signal channels based upon informationderived from the modulated tones.
 48. The method of claim 47 wherein thepower output is changed by changing the applied bias to a channel activeelement.
 49. The method of claim 48 comprising the further step ofmaintaining the current power level of each channel signal so that allthe channel power outputs are substantially the same.
 50. The method ofclaim 44 comprising the further step of changing the current emissionwavelength of a modulated source to a predetermined emission wavelengthin response to a determination that the current emission wavelength isoffset from the predetermined emission wavelength for that signalchannel.
 51. The method of claim 50 wherein the step of changing thecurrent emission wavelength of the modulated source is accomplished bychanging a local operational temperature of the modulated source. 52.The method of claim 44 comprising the further steps of: changing anapplied bias to the active element to maintain the current power levelof a channel signal at a predetermined power level in response to adetermination that the current power level is offset from apredetermined power level for that signal channel; and changing thecurrent emission wavelength of the modulated source to a predeterminedemission wavelength in response to a determination that the currentemission wavelength is offset from the predetermined emission wavelengthfor that signal channel.
 53. A wavelength locking and power controlfeedback loop for a multi-channel optical transmitter, the loopcomprising: a plurality of lasers coupled to a plurality of opticalchannels; an array of channel tagging elements, coupled to receive aplurality of optical signals on the plurality of optical channels, thatinsert a channel identifier tag on at least two optical signals withinthe plurality of optical signal; a multiplexer that combines theplurality of optical signals, including the at least two tagged opticalsignals, into an optical signal group; a wavelength locking sub-module,coupled within the feedback loop and to receive at least a portion ofthe optical signal group, that splits a first optical signal within theoptical signal group into first and second locking signals derived fromthe channel identifier tag and identifies a wavelength offset byanalyzing the first and second locking signals within the electricaldomain; a signal processing sub-module, coupled within the feedback loopand to the wavelength locking sub-module, that generates an error signalthat contains information to compensate for the identified wavelengthoffset; and a switch, coupled to receive the error signal, that switchesthe error signal using information derived from the channel identifiertag so that a wavelength offset or power level is adjusted on the firstoptical signal.
 54. The feedback loop of claim 53 wherein the array ofchannel tagging elements is an array of elements selected from a groupconsisting of: front PINs, semiconductor optical attenuators, variableoptical attenuators, and a laser modulator.
 55. The feedback loop ofclaim 53 wherein the channel identifier tag identifies a particularoptical channel within the plurality of optical channels by inserting aunique tone frequency on each of the at least two optical signals thatis lower than the frequencies of the at least two optical signals. 56.The feedback loop of claim 55 further comprising: a first lowpassphotodetector, coupled within the wavelength locking sub-module, thatgenerates the first locking signal from the unique tone frequency; and asecond lowpass photodetector, coupled within the wavelength lockingsub-module, that generates the second locking signal from the uniquetone frequency.
 57. The feedback loop of claim 55 wherein the wavelengthlocking sub-module further identifies the wavelength offset inaccordance with a desired carrier frequency separation across theplurality of optical channels.
 58. The feedback loop of claim 57 whereinthe wavelength locking sub-module uses an intersection between alockpoint on a rising side of the second locking signal and atemperature sensitive ITU optical carrier grid frequency to furtheridentify the wavelength offset.
 59. The feedback loop of claim 53wherein the error signal represents a mean optical carrier frequencydeviation of the first optical signal relative to a known calibratedfrequency target of the first optical signal.
 60. The feedback loop ofclaim 53 wherein the error signal is provided to a first controller,associated with a first laser within the plurality of lasers, in orderto adjust a bias current driving the first laser.